1. Field of the Invention
The present invention relates in general to a steering control system for a four-wheel-steerable motor vehicle, and more particularly to a control system for controlling the steering angle of the rear wheels of a four-wheel-steerable motor vehicle.
2. Description of the Prior Art
In a regular motor vehicle wherein only the front wheels are steerable, the yaw rate gain and the phase angle relative to the steering frequency (f) have the characteristics respectively depicted by dashed lines "a" and "b" of the graphs of FIGS. 3A and 3B. As can be seen from these curves, with an increase in the steering frequency (f), the yaw rate gain is reduced and the phase lag of the yawing, is increased. This means that during steering at a higher frequency, the yaw response inevitably is poor.
In view of this drawback, a so-called four-wheel-steerable motor vehicle has been proposed in which the rear wheels as well as the front wheels are steered in accordance with the operation of a steering wheel. In this type of motor vehicle, the steering angle of the rear wheels is usually determined in the following manner.
The motion of the motor vehicle is represented by the following four equations (1), (2), (3) and (4): EQU M(y+V.phi.)=Ff+Fr . . . (1)
where:
M=mass of vehicle, PA0 y=transverse acceleration, PA0 V=vehicle speed, PA0 .phi.=yaw angular speed, PA0 Ff=side force of front wheels, and PA0 Fr=side force of rear wheels. EQU .phi.=aFf-bFr . . . (2) PA0 I=yaw inertia moment of vehicle PA0 .phi.=yaw angular acceleration, PA0 a=distance between front axle and center of gravity of vehicle, PA0 b=distance between rear axle and center of gravity of vehicle. ##EQU2## where: Cf=cornering power of front wheels, PA0 Cr=cornering power of rear wheels, PA0 .delta.f=steering angle of front wheels, (=steering angle .theta. of steering wheel/steering gear ratio (N)), PA0 .delta.r=steering angle of rear wheels, and PA0 y=transverse acceleration of vehicle. PA0 .delta.f(s)=Laplace transform of front wheel steering angle, PA0 s=Laplace transform variable, PA0 l=Wheel base (= a+b), PA0 .delta.(s)=Laplace transform of rear wheel steering angle, PA0 .omega.n=natural frequency, PA0 .zeta.=damping ratio, ##EQU4## PA0 .delta.r(s): Laplace transform of rear wheel steering angle; PA0 A: function A(V) of vehicle speed V; PA0 B: function B(V,.phi..sub.0) of vehicle speed V and normal yaw rate .phi..sub.0 ; PA0 C: function C(V,.phi..sub.0) of vehicle speed V and normal yaw rate gain .phi..sub.0 ; PA0 D: function D(V,.phi..sub.0) of vehicle speed V and yaw rate gain .phi..sub.0 ; and PA0 S: Laplace transform variable.
where:
The above-mentioned four equations (1), (2), (3) and (4) are combined by using Laplace transformation to provide the following equation. ##EQU3## where: .phi.(s)=Laplace transform of yaw angular speed,
Hitherto, the value of .delta.r(s)/.delta.f(s) is replaced with K+.tau..sub.1 S (wherein, K and .tau..sub.1 are constants in order to determine the steering angle of the rear wheels by finding the values of K and .tau..sub.1 so as to make the value .phi.(s)/.delta.f(s) equal another constant k.
As is seen from the yaw rate gain characteristic depicted by the line "c" of FIG. 3A and the phase angle characteristic depicted by line "d" of FIG. 3B, the above-mentioned method can prevent the lowering, of the yaw rate gain and the yaw response lag at higher steering frequencies.
However, the above-described has the following drawbacks.
In the conventional method, to achieve constants the above-mentioned characteristics, "K" and ".tau..sub.1 " are employed and thus the value "k" is expressed as k=-(bCr.tau..sub.2)/I.
Thus, the normal yaw rate gain .phi..sub.0 can not be changed. This means that, as shown by curve "e" of FIG. 3A, the tuning of the normal yaw rate gain (the steerability) for each motor vehicle can not be achieved. As is well known, if the yaw rate gain can not be tuned, it is impossible to prevent the undesired lowering of the yaw rate gain at higher steering frequencies while keeping the normal yaw rate gain at its original value.